1. Field of the Invention
This invention relates to liquid crystal displays. More particularly, this invention relates liquid crystal displays having reduced photo leakage current and improved picture quality.
2. Discussion of the Related Art
Cathode ray tubes (CRT) are widely used display devices in television sets, measurement instrumentation, and information terminals. However, because they are heavy and consume significant power, CRTs are not well suited to applications that require compact, lightweight, low power displays.
A substitute for CRTs, liquid crystal displays, is compact, lightweight, and consumes low power. A liquid crystal display device incorporates a matrix of liquid crystal cells that are sequentially selected, line-by-line, to produce picture information. To do so, liquid crystal cells vary their light transmittance in accord with data signals that carry picture information.
A liquid crystal display includes a liquid crystal panel having liquid crystal cells and driver integrated circuits (IC) for driving those liquid crystal cells. A liquid crystal panel is usually comprised of a thin film transistor array substrate and of a color filter substrate that are disposed in a facing relationship. Additionally, a liquid crystal layer is interposed between the thin film transistor array substrate and the color filter substrate.
A thin film transistor array substrate includes a plurality of data lines for transmitting data signals from data driver integrated circuits to the liquid crystal cells, and a plurality of gate lines for transmitting scan signals supplied from gate driver integrated circuits to the liquid crystal cells. The liquid crystal cells are defined by intersections of the data lines and the gate lines. As a gate driver integrated circuit sequentially supplies scan signals to the gate lines, data signals are supplied to the liquid crystal cells.
A common electrode on the color filter substrate and pixel electrodes on the thin film transistor array substrate are used to produce electric fields across the liquid crystal layer. By controlling the voltage applied to the pixel electrodes the light transmittance of each liquid crystal cell can be controlled.
To control the voltage applied to a pixel electrode, a thin film transistor is formed in each liquid crystal cell. When a scan signal is supplied to a gate electrode of the thin film transistor, a conductive channel is formed between a source electrode, which is connected to a data line, and a drain electrode, which is connected to an associated pixel electrode. Thus, the thin film transistor controls the flow of data signals to the pixel electrodes. Thin film transistors usually use amorphous silicon, which can be formed at a low temperature on a large-scale insulation substrate, such as a low-priced glass substrate, as an active layer. Accordingly, controlling data signals applied to each liquid crystal cell by selective switching of the thin film transistors, the light transmittance of the liquid crystal cells can be controlled.
The light transmitting process of a liquid crystal display device will now be described. First, a common electrode voltage is supplied to the common electrode. Then, scan signals are sequentially supplied to the gate lines by gate driver integrated circuits. The scan signals are applied to the gate electrodes of the thin film transistors. Meanwhile, data signals are supplied to the liquid crystal cells by data driver integrated circuits via data lines. The data signals are applied to the source electrodes of the thin film transistor.
Accordingly, the data signals are supplied to the drain electrodes through conductive channels formed when a scan signal is applied to a particular transistor. The data signal is supplied to the drain electrode through the channel. The data signal is thus supplied to the pixel electrode that is connected to the drain electrode. In practice, the pixel electrode is also connected to a storage electrode. Thus, the data signal voltage supplied to each pixel electrode is stored in a storage electrode. When a thin film transistor is turned off, the voltage across its storage capacitor continues to be applied to the pixel electrode, thereby maintaining the liquid crystal cell drive.
As mentioned, since a common electrode voltage is applied to the common electrode, and a data signal voltage is applied to the pixel electrode, electric fields are produced across the liquid crystal layer by the potentials of the common electrode and of the pixel electrodes.
When an electric field is applied across the liquid crystal layer, the liquid crystal is rotated by dielectric anisotropy to selectively transmit light emitted from a back light unit through the pixel electrode. The electric field strength is controlled by the data signal voltage that is applied to the pixel electrode, and the light transmittance of the liquid crystal layer is controlled by the electric field strength.
Unfortunately, continuous applying a single electric field polarity degrades the liquid crystal. To prevent such degradation the data signals alternately switch polarity relative to the common voltage. This general technique is called inversion driving.
FIG. 1 is an exemplary view showing voltage waveforms applied to a liquid crystal display device. As shown, a common electrode voltage (Vcom) is applied to the common electrode, while a data signal voltage (VDATA) is applied to the source electrode of a thin film transistor via the data line. Additionally, a scan signal (VG) is applied to the gate electrode of the thin film transistor via the gate line.
During the turn-on period of the thin film transistor, when the scan signal (VG) is applied at a high potential, the positive data signal voltage (VDATA) is supplied to the pixel electrode and to the source capacitor by the drain electrode. At that time, the positive data signal voltage (VDATA) is charged into the storage capacitance. Thus, as shown, a pixel electrode voltage (VP) is produced.
When the thin film transistor is turned off by removal of the scan signal (VG), a voltage drop from the charged pixel electrode voltage (VP) occurs because of a parasitic capacitance. The voltage drop is called a kick-back voltage (“ΔVP”), reference FIG. 1.
During the turn-off period the pixel electrode voltage (VP) charged into the storage capacitor is applied to the pixel electrode, thus maintaining drive to the liquid crystal cell.
Meanwhile, in the n+1th frame, since the above-described inversion driving method is used, a negative data signal voltage (VDATA) is supplied through the source and drain electrodes to the pixel electrode and to the storage capacitor. Accordingly, as shown in FIG. 1, the pixel electrode voltage (VP) in the n+1th frame has a voltage waveform that is symmetrical relative to the common electrode voltage (Vcom) with the pixel electrode voltage (VP) of the nth frame,
Meanwhile, since the thin film transistor channel is amorphous silicon, if an external light is irradiated onto the channel a photo-induced leakage current results. The photo-induced leakage current decreases the voltage of the storage capacitor during the turn-off period, which reduces the pixel electrode voltage (VP) as shown in FIG. 1.
Since a transmission type liquid crystal display device does not emit light, it requires an optical source such as a back light unit or external light. A liquid crystal display device that uses a back light unit is called a transmission type liquid crystal display device, while a liquid crystal display device that uses external natural light is called a reflective type liquid crystal display device.
The transmission type liquid crystal display device usually locates the back light unit either below the liquid crystal display panel or along an edge. Currently, the edge-type transmission type is more common.
However, the transmission type liquid crystal display device is inefficient in that only 3% to 8% of the light from the back light unit is actually transmitted. For example, using the reasonable assumptions that the transmittance of two polarization plates is about 45%, that the transmittance of two glass substrates is about 94%, that the transmittance of a thin film transistor array and pixel is about 65%, and that the transmittance of a color filter is about 27%, then the overall transmittance of a liquid crystal display device is about 7.4%.
Thus, the amount of light from a transmission type liquid crystal display device is only about 7% of the light from the back light unit. Thus, if a high luminance is required the back light unit needs to be very bright, something that causes great power consumption. Thus, in order to supply enough power to the back light unit a large, heavy, high capacity battery is required. Even with such a battery there is a limit on how long the liquid crystal display device can be used while traveling. Further, such a large capacity battery is an obstacle to achieving the desired size, weight, and portability.
A solution to the power problems of the transmission type liquid crystal display device is the reflective type liquid crystal display device. The reflective type liquid crystal display produces an image using external light. Thus, only a small amount of power is required. Accordingly, a reflective type liquid crystal display device can be used for extended periods of time, is more compact, lightweight and portable. Furthermore, since the entire unit pixel can be used, the aperture ratio of a reflective type liquid crystal display device is excellent.
The reflective type liquid crystal display device includes a translucent reflective electrode that is made of a light reflective metal, instead of the transparent conductive material used in a transmission type liquid crystal display device. The reflective electrode produces an electric field across the liquid crystal layer in conjunction with a common transparent electrode on the color filter substrate.
When an electric field is applied across the liquid crystal layer, the liquid crystal is rotated by the dielectric anisotropy. This controls the amount of external light that is transmitted through the color filter substrate, and thus the amount of light reflected by the reflective electrode. The reflected light is thus controlled by voltages applied to the reflective electrodes.
However, the reflective type liquid crystal display device has a problem in that since the materials of the reflective electrode and the common transparent electrode are different, the driving characteristics of the liquid crystal are deteriorated, which results in degradation of the image produced on the liquid crystal display device.
In addition, the external light required for the reflective type liquid crystal display device is not constant. That is, while a reflective liquid crystal display device can be used during the day or when artificial light exists, it will not work in the dark.
Consequently, transmission/reflective type liquid crystal display devices have been proposed. The transmission/reflective type liquid crystal display device adopts a reflection mode when external light is available, but a transmission mode when external light is not available.
The transmission/reflective type liquid crystal display device will be described with reference to the accompanying drawings. FIG. 2 shows a plan view of a unit pixel of a transmission/reflective type liquid crystal display device. With reference to FIG. 2, gate lines 104 are arranged at regular intervals on a substrate, and data lines 102 are arranged at regular intervals, but in a crossing relationship. Accordingly, the gate lines 104 and the data lines 102 form a matrix of liquid crystal cells. A thin film transistor (TFT), a reflective electrode 114 and a pixel electrode 115 are provided in each liquid crystal cell.
Each thin film transistor includes a gate electrode 110 that extends from a gate line 104, and a source electrode 108 that extends from a data line 102 and that overlaps the gate electrode 110. Additionally, each thin film transistor includes a drain electrode 112 that corresponds to the source electrode 108 on the gate electrode 110. Each thin film transistor (TFT) also includes an active layer (not shown in FIG. 2) for forming a conductive channel between the source electrode 108 and the drain electrode 112 when a scan signal is supplied to the gate electrode 110. As the active layer, amorphous silicon is beneficial in that it can be formed at a low temperature on a low-priced glass substrate.
Extending the conductive channel tends to improve the characteristics of the thin film transistor (TFT). Thus, the conductive channel is preferably formed in an “L” shape or in a “U” shape. FIG. 2 illustrates the “U” shape. To achieve a “U”-shaped conductive channel, the source electrode 108 extends with a hook shape from the data line 102, and the drain electrode 112 is inside the hook.
Compared to an “L”-shaped conductive channel, the “U”-shaped conductive channel is longer. Furthermore, the overlap between the drain electrode 112 and the gate electrode 110 can be formed despite some misalignment in the fabrication process. However, the overlap between the drain electrode 112 and the gate electrode 110 is significantly influenced by misalignment. Thus, the parasitic capacitance (Cgd) between the drain electrode 112 and the gate electrode 110 can be changed enough that picture quality degradation results.
The pixel electrode 115 and the drain electrode 112 electrically connect through a drain contact hole 116 that is formed through an insulation film (not shown in FIG. 2). The pixel electrode 115 is formed in the pixel region of each liquid crystal cell and is comprised of a transparent conductive material.
At marginal portions of the each pixel region is a translucent reflective electrode 114 that is comprised of a highly reflective and conductive material. The reflective electrode 114 is overlapped by the insulation film and by the pixel electrode 115. The reflective electrode 114 is also formed near the thin film transistor (TFT). Thus, the reflective electrode acts as a shielding plate 114A that blocks light that is directed toward the conductive channel of the thin film transistor (TFT). Because of the shielding plate 114A, light irradiated toward the conductive channel is blocked, and thus photo-induced current leakage of the thin film transistor (TFT) is reduced or prevented.
Referring once again to FIG. 1, by minimizing the reduction of the pixel electrode voltage (VP) charged into the storage capacitor during the turn-off period of the thin film transistor (TFT), the picture quality of the liquid crystal display device could be improved.
In conventional transmission/reflective type liquid crystal displays of the type used in notebook personal computers, if the external illumination is below about 50,000 to 60,000 (Lux), that light is blocked by the shielding plate 114A. Referring now to FIG. 3, but if the external illumination is around 100,000˜110,000 (Lux), something like noon sun light, incident light having an incident angle of less than 30° relative to the display unit of the notebook type personal computer is not blocked. Problems can arise because the shielding plate 114A does not sufficiently block the light.
Referring once again to FIG. 2, if the shielding plate 114A fails to sufficiently block the light incident at regions “A” to “D” photo-induced leakage currents and their consequent picture quality degradation can result.
FIG. 4 is a sectional view of the thin film transistor taken along line I–I′ of FIG. 2. As shown, the sectional structure includes the gate electrode 110 on a substrate 101, and a gate insulation film 130 over the substrate 101 and over the gate electrode 110. Beneficially, the gate electrode 110 is formed along with the gate line 104.
Still referring to FIG. 4, an active layer 136 is on the gate insulation film 130 and over the gate electrode 110. The active layer 136 includes an amorphous silicon semiconductor layer 132 and an n+ amorphous silicon ohmic contact layer 134 that is highly doped with phosphor. The drain electrode 112 is located above the center of the active layer 136, while the source electrode 108, which is also above the active layer 136, is located away from the drain electrode 112 toward the edges of the active layer 136.
The ohmic contact layer 134 is partially removed during patterning of the source electrode 108 and the drain electrode 112 to assist defining the “U” shaped channel.
Still referring to FIG. 4, a passivation film 138 is formed over the source electrode 108 and the drain electrode 112, over the active layer 136, and over the gate insulation film 130. The passivation film 138 can be an inorganic insulation film, such as SiNx or SiOx, or to improve the aperture ratio, the passivation film 138 can be an organic insulation film such as benzocyclobutane (BSB), spin-on-glass (SOG) or acryl having a low dielectric constant. If the reflective electrode 114 (described in more detail subsequently) is directly deposited on an organic insulation film, to prevent contamination of the deposition chamber by organic materials, the passivation film 138 can be formed by stacking organic and insulation films.
Still referring to FIG. 4, the reflective electrode 114 and the shielding plate 114A are on the passivation film 138. Those structures can be simultaneously patterned. Then, an inorganic insulation film 140, such as SiNx or SiOx, is formed over the passivation film 138 and over the reflective electrode 114. The inorganic insulation film 140 electrically insulates the reflective electrode 114 and a pixel electrode 124 (described below) such that when an electric field is applied between the common electrode (not shown) on the transparent color filter substrate and the translucent reflective electrode 114, deterioration of the liquid crystal due to different materials is prevented.
Still referring to FIG. 4, a drain contact hole 116 is then formed through the passivation film 138 and through the inorganic insulation film 140 so as to expose a portion of the drain electrode 112. Then, the pixel electrode 124 is formed over the inorganic insulation film 140 and into the drain contact hole 116 so as to contact the drain electrode 112.
Referring now back to FIG. 2, the gate lines 104 extend perpendicular to the gate electrodes 110. The gate lines 104 act as a first electrode of a storage capacitor. An insulation film overlaps the first electrode, and then a storage electrode (not shown) overlaps the first electrode with the insulation film acting as a dielectric layer, thus forming a storage capacitor 118. The storage electrode 118 connects to a pixel electrode 115 through a storage contact hole 122.
FIG. 5 is a cross-sectional view of the storage capacitor 118 taken along line II–II′ of FIG. 2. As shown, the storage electrode includes the first electrode 119 on the substrate 101. The gate insulation film 130 covers the substrate 101 and the first electrode 119. The storage electrode 120 is formed on the gate insulation film 130 and over part (reference FIG. 2) of the first electrode 119.
Still referring to FIG. 5, the passivation film 138 and the inorganic insulation film 140 are stacked over the gate insulation film 130 and over the storage electrode 120. The reflective electrode 114 is formed as previously described. A storage contact hole 122 is formed through the inorganic insulation film 140 and through the passivation film 138 to expose a portion of the storage electrode 120. A pixel electrode 124 is then formed over the inorganic insulation film 140, into the storage contact hole 122, and in electrical contact with the storage electrode 120.
Accordingly, the storage electrode 120 overlaps the first electrode 119 with an interposed gate insulation film 130, thereby forming the storage capacitor 118.
The storage capacitor 118 is charged to a data signal voltage during the turn-on period of the thin film transistor (TFT) (when a scan signal is applied to the gate line 104). The charged voltage is then applied to the pixel electrode 124 during the turn-off period of the thin film transistor (TFT), thereby maintaining the state of the liquid crystal in the OFF period.
As mentioned, a conventional liquid crystal display device in a notebook type personal computer can have a degraded picture when light is incident at an angle of less than 30°. This is because the thin film transistor conductive channel receives such incident light, which creates photo-induced leakage current, which causes picture quality degradation.
Therefore, a new liquid crystal display that does not suffer from photo-induced leakage current when light is incident at an angle of less than 30° would be beneficial.